Burial of cables may be an efficient way of protecting submarine cables in areas where the cables may be damaged by trawling, ship anchors, and other bottom threats. A new generation of arrays are being developed for applications, including environmental monitoring, scientific observations of the ocean in littoral areas, sub-bottom mapping for hydrocarbon searches and well depletion monitoring, and surveillance applications. These arrays may typically be 100 m to 1000 m long, less than ¾ inch in diameter and may include multiple in-line sensors along their length. Typically it may be desirable to bury these arrays several inches below the sea floor to increase their survivability, often 4 inches to 8 inches below the sea floor. Often, these arrays may be deployed in water from 30 m to 500 m deep with up to 15 degree bottom slopes and in soils with support capabilities as low as 0.5 psi. These arrays may be able to withstand ocean bottom currents of one knot and go over seabed obstacles as high as three feet.
Submarine telecommunications cables, power cables and many of the U.S. Navy's cables and arrays are commonly buried in waters up to 1,500 meters deep, and even deeper in some areas of the world. In current systems, one of two main burial methods may be used. A first type of system uses a plow to simultaneous lay and bury the cable. The second type of system uses a Remotely Operated Vehicle (“ROV”) with a jetting system to bury the cable after it has been laid (i.e., post-lay burial operation).
Plowing is the dominant burial technique used for submarine cables. Current plows are large structures weighing 10 to 20 tons and requiring typical pulling tensions of 40 to 60 tons in order to bury cables 1.0 to 1.5 m (and deeper in soft soils) below the seabed. The main advantages of using a towed plow simultaneously with the cable lay are good work rate, instantaneous and effective protection, and high reliability. A plow is not generally appropriate for post-lay operations due to the intrinsic limitations of low maneuverability and the need for loading and unloading the cable on the seabed. Also, since a plow depends on ground reaction forces for stability, steep surfaces and/or soft, unstable bottoms can induce the plow to tip over and/or run away. Since the cable is threaded through the structure of the plow, instability in these situations may lead to danger to both cable and plow. Experience to date shows that plows tends to be more efficient in soils with a slope of 5 degrees or less.
ROVs are mostly used to bury cables during post-lay inspection operations. In contrast to a plow, an ROV equipped with a jetting tool for cable burial is able to swim along and above the cable route and can work in areas with steeper slopes. While the ROV has better maneuverability than the plow, with a jetting ROV it is difficult to reliably bury the cable to the specified depth. Since the ROV is hovering above the cable it is difficult to maintain a fixed distance between the jetting tool and the cable. Consequently, the work rate of an ROV tends to be slower than that of the plow since there is often the need to have the ROV perform multiple passes over the cable to achieve the desired burial depth. Additionally, the presence of strong bottom currents can impose limitations in the use of an ROV in deep water.
While existing ROV and plow technologies commonly used to bury submarine cables are well tested, there are key and critical differences between the burial of conventional submarine cables and the arrays addressed herein. First, conventional plows and jetting ROVs are connected to a surface vessel which supplies all the power needed by the plow and/or ROV. Because of this relatively unlimited power supply, plows and jetting ROVs are not designed for optimum burial efficiency per unit of power used. Rather, plows and jetting ROVs are mainly designed to achieve maximum and reliable burial depth and fast rates of burial. Typical pulling tensions on the plows are 40 to 60 tons and power requirements for jetting ROVs are several hundred kilowatts. This represents two to three orders of magnitude more power than could be practically used by an ABV to bury the new generation of arrays that may be buried between 4 and 8 inches below the seabed.
There is a need for an ABV that is a totally autonomous, low cost vehicle, which must reliably bury a cable up to 1.0 km long. Since final cost of the system may be a key driving factor, the ABV must bury the cable and navigate the entire route using the least amount of energy possible. The smaller the energy footprint, the smaller and more economical the vehicle will be. Simply scaling down a conventional cable burial system will not provide a viable solution, and a new design needs to be developed that meets the specific requirements. Overall, the existing technology for cable burial does not provide design experience or performance data applicable to the development of the needed ABV.